Capacitive micromachined ultrasonic transducers (CMUTs) have been increasingly used in medical imaging applications. For example, CMUTs have been able to improve medical ultrasound imaging probes and to provide high-intensity focused ultrasound for use in medical therapy.
Originally, CMUTs were produced using a typical MEMS manufacturing technique in which a release layer is etched out, leaving a free-standing membrane. Such CMUT devices can be produced directly on a silicon substrate or on top of the control circuitry.
An alternative CMUT process involves the bonding of a silicon-on-insulator (SOI) wafer. This wafer is used to form a single crystal silicon membrane after the handle wafer and the buried oxide layer are stripped. The benefits of this approach are process simplicity and the higher Q of the single crystal Silicon membrane. Such CMUTs can be produced as stand alone devices but cannot be easily integrated on top of the support circuitry. Although the metal interconnect layers of the support circuitry afford the opportunity of easily contacting the bottom CMUT electrodes, the primary challenge is producing a coplanar surface on which the SOI wafer is subsequently bonded. Even in the case where the CMUT is produced on a silicon substrate, contact to the bottom electrodes has been problematic. Conventional methods for providing such contact involve the use of through-silicon vias (TSVs). However, the use of TSVs requires multiple process steps including deep silicon etching using a Bosch etch process, which is relatively slow and expensive.